Diagnosis & Repair

WARNING: This page is about a different car, the 2002 Toyota Prius. However, it is still accessible from the selected car via links, so may be relevant.

Using Toyota hand-held tester or scan tool, check for any other DTCs. If only DTC P0420 exists, go to next step. If other DTCs exist, diagnose and repair those DTCs first and then retest.
Check exhaust system for leaks. If no exhaust leak exists, go to next step. If exhaust leak exists, repair exhaust system as necessary.
Check heated oxygen sensor No. 1. See DTC P0130: HEATED OXYGEN SENSOR NO. 1 CIRCUIT . If heated oxygen sensor No. 1 operates properly, go to next step. If heated oxygen sensor No. 1 does not operate properly, repair or replace components as necessary.
Check heated oxygen sensor No. 2. See DTC P0136: HEATED OXYGEN SENSOR NO. 2 CIRCUIT test. If heated oxygen sensor No. 2 operates properly, replace catalytic converter. If heated oxygen sensor No. 2 does not operate properly, repair or replace components as necessary.

Fig 1: Identifying Correct Waveform Pattern For Heated Oxygen Sensor No. 1 & Incorrect Waveform Pattern For Heated Oxygen Sensor No. 2
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